The present invention relates to an LED array constituted by forming a plurality of LEDs (light emitting diodes) on a single semiconductor substrate, and an LED printer head constituted by employing this LED array, and in particular, it relates to an LED array that is capable of supporting a high density printer head of 1200 DPI (dots per inch) or higher.
Light emitting diode (hereafter referred to simply as LED) arrays are employed as exposure light sources (print heads) for the photosensitive drums in electrophotographic printers. FIG. 1 illustrates an example of an LED structure in the prior art, with FIG. 1(a) presenting its top view and FIG. 1(b) presenting a cross section through line A-Axe2x80x2 in FIG. 1(a). The LED array illustrated in FIG. 1 supports a low density printer head with a resolution of 600 DPI or lower, and is constituted by simply providing LEDs 10 in a single row on an n-type semiconductor substrate 2.
In FIG. 1, a plurality of p-type semiconductor layers 13 are formed on the n-type semiconductor substrate 2, with a layer insulating film 12 having opening portions 16 formed at the surface of the n-type semiconductor substrate 2. On the layer insulating film 12, a plurality of p-side electrodes (discrete electrodes) 14 that are individually connected to a p-type semiconductor layer 13 at an opening portion 16 are formed. In addition, an n-side electrode (common electrode) 15 is formed over the entire rear surface of the n-type semiconductor substrate 2. When a voltage is applied between a p-side electrode 14 and the n-side electrode 15 at an LED 10, a phenomenon of light emission occurs at the bonding surface of the n-type semiconductor substrate or interface of the n-type semiconductor layer 13, and the emitted light is radiated to the outside through the surface of the p-type semiconductor layer 13. The p-side electrodes 14 are each constituted of an aluminum (Al) film or an Al alloy film, whereas the n-side electrode 15 is constituted an Au alloy film.
However, in a super-high density LED array of 1200 DPI or higher, the pitch of the p-side electrodes becomes narrower and the wiring space for the p-side electrodes becomes reduced, making it difficult to provide a bonding pad (p-side pad electrode) for each p-side electrode. Thus, an LED array supporting a high density of 1200 DPI or higher designs the structure illustrated in FIG. 2 to reduce the number of p-side pad electrodes. FIG. 2(a) is a top view illustrating an example of an LED array in the prior art the supports a high density of 1200 DPI. FIG. 2(b) presents a cross section through line A-Axe2x80x2 in FIG. 2(a), and FIG. 2(c) presents a cross section through line B-Bxe2x80x2 in FIG. 2(a).
The LED array illustrated in FIG. 2 is achieved by providing a plurality of LEDs at each of a plurality of n-type semiconductors blocks 11 with the elements isolated from one another by a high resistance semiconductor substrate 32 and separating grooves 3. In each n-type semiconductor block 11, a plurality of p-type semiconductor layers 13, p-side electrodes 44 to be individually connected to the p-type semiconductor layers 13, an n-type contact electrode 45a to be connected to the n-type semiconductor block 11 and an n-side pad electrode 45b to be connected to the n-side contact electrode 45a are formed. Among the plurality of p-side electrodes 44 provided in the block, only a specific number of p-side electrodes are provided with a p-side pad electrode 44b (in FIG. 2, one p-side pad electrode 44b is formed in each block). An n-side electrode 45 constituted of the n-side contact electrode 45a and the n-side pad electrode 45b is a common electrode shared by all the LEDs within the block.
When a voltage is applied between a p-side electrode 44 and the n-side electrode 45, an LED causes a light emission phenomenon at the bonding surface of the n-type semiconductor substrate 32 and the p-type semiconductor layer 13, and this emitted light is radiated to the outside through the surface of the p-type semiconductor layer 13. The p-side electrodes 44 are each constituted of an aluminum (Al) film or an Al alloy film, whereas the p-side electrode 45 is constituted of an Au alloy film.
Furthermore, p-side common wirings 4 which connect with specific p-side electrodes 44 in the individual blocks at via holes 21 are formed, and through the p-side common wirings 4, p-side electrodes 44 that are not provided with a p-side pad electrode in a given block are connected to p-side electrode 44 having a p-side pad electrode in another n-type semiconductor block 11. A first layer insulating film 12 is formed between the n-type semiconductor blocks 11 and the p-side common wirings 4, and a second layer insulating film 48 is formed between the p-side common wirings 4 and the p-side electrodes 44.
However, in LED arrays in the prior art, since it is necessary to separately form the p-side electrodes and the n-side electrodes using different conductive film materials, the number of manufacturing steps is high, presenting a problem in that the production cost is high also. In particular, in the LED array illustrated in FIG. 2, which necessitates the n-side contact electrodes and the n-side pad electrodes to be formed separately, the number of manufacturing steps is further increased, to result in a further increase in production cost.
Accordingly, an object of the present invention is to provide a new and improved LED array, whose n-side contact electrodes and p-side electrodes, n-side contact electrodes and n-side pad electrodes or n-side contact electrodes, n-side pad electrodes and p-side electrodes can be formed using a single conductive film through a single manufacturing step to simplify the manufacturing process, to achieve a reduction in production cost and also to achieve a reduction in the inconsistency in characteristics among the individual substrates (wafers), and an LED printer head employing the LED array.
Another object of the present invention is to provide a new and improved LED array that achieves higher yield with peeling of the n-side pad electrodes prevented by forming the n-side pad electrodes at the n-side semiconductor substrate in close adherence, and an LED printer head employing the LED array.
Yet another object of the present invention is to provide a new and improved LED array that achieves a simplification in the manufacturing process and reduced production cost by constituting the n-side electrodes in a single-layer structure, and an LED printerhead employing the LED array.
A still further object of the present invention is to provide a new and improved LED array that achieves a reduction in the chip size by constituting the n-side electrodes in a simple graphic shape and the entire n-side electrodes as n-side pad electrodes, and an LED printer head employing the LED array.
A still further object of the present invention is to provide a new and improved LED array that can be achieved as an end surface light emitting LED array by providing the p-side pad electrodes and the n-side electrodes at the same side relative to the row of the light emitting portions that are formed at high density, to facilitate mounting at a printer head and to achieve a reduction in the width of the LED array, and an LED printer head employing the LED array.
A still further object of the present invention is to provide a new and improved LED array, and an LED printer head through which the head size of the printer head can be reduced by constituting the printer head employing the LED array described above.
In order to achieve the objects described above, in the LED array in a first aspect of the present invention, which is constituted by forming a second conductive-type semiconductor layer at a first conductive-type semiconductor substrate and forming a first conduction-side contact electrode to be connected to the semiconductor substrate and a second conduction-side electrode to be connected to the semiconductor layer at the surface of the semiconductor substrate on the side where the semiconductor layer is formed, the first conduction-side contact electrode and the second conduction-side electrode are constituted of one and the same conductive film material.
In the LED array in a second aspect of the present invention, which is achieved by forming a second conductive-type semiconductor layer on a first conductive-type semiconductor substrate and forming a first conduction-side electrode constituted of a first conduction-side contact electrode to be connected to the semiconductor substrate and a first conduction-side pad electrode to be connected to the first conduction-side contact electrode at the surface of the semiconductor substrate on the side where the semiconductor layer is formed, the first conduction-side electrode achieves a single-layer structure in which the first conduction-side contact electrode and the first conduction-side pad electrode are formed as an integrated electrode by using a single conductive film. It goes without saying that the first conduction-side contact electrode, the first conduction-side pad electrode and the second conduction-side electrode may be formed from a single conductive film. In addition, an Au film or an Au alloy film, for instance, may be used to constitute the conductive film.
In the LED array in a third aspect of the present invention, which is achieved by forming a second conductive-type semiconductor layer at a first conductive-type semiconductor substrate and forming a first conduction-side electrode which includes a first conduction-side pad electrode for connection with an external circuit is to be connected to the first conductive-type semiconductor substrate at the surface of the semiconductor substrate on the side where the semiconductor layer is formed, the first conduction-side pad electrode is formed and tightly bonded at the surface of the semiconductor substrate.
To be more specific, a structure achieved by, for instance, forming a layer insulating film having an n-side opening portion at the surface of a semiconductor substrate and forming a first conduction-side electrode that includes a first conduction-side pad electrode within the n-side opening portion to bond the entire first conduction-side electrode to the surface of the semiconductor substrate may be adopted. The first conduction-side electrode may adopt a single-layer structure constituted of one layer of conductive film so that the entire first conduction-side electrode constitutes the first conduction-side pad electrode. In addition, the first conduction-side electrode achieving the single-layer structure and the second conduction-side electrode may be constituted of the same conductive film material.
Moreover, in the LED array in a fourth aspect of the present invention, which is achieved by forming N (N is an integer equal to or larger than 2) light emitting portions constituted of a second conductive-type semiconductor layer in a single row at a first conductive-type semiconductor substrate at the substrate surface and forming a first conduction-side electrode that is to constitute a pad electrode to connect the semiconductor substrate with an external circuit and N second conduction-side electrodes that are to be individually connected with the light emitting portions on a surface of the semiconductor substrate with at least one of the N second conduction-side electrodes provided with a second conduction-side pad electrode for connection with an external circuit; the first conduction-side electrode is formed on the same side as the second conduction-side pad electrode relative to the row of light emitting portions and is connected with the semiconductor substrate at the same side as the second conduction-side pad electrode.
More specific examples of the LED array described above include, for instance, a matrix type LED array achieved by providing, in a single row, a plurality of semiconductor blocks constituted of the first conductive-type semiconductor substrate provided with the light emitting portions, the first conduction-side electrode and the second conduction-side electrodes, with the elements isolated from one another and forming common wirings for connecting specific second conduction-side electrodes formed in different semiconductor blocks.
Moreover, in the LED printer head according to the present invention, which is provided with the LED array described above and a drive circuit for driving the LED array, the wire bonded to the first conduction-side electrode and the wire bonded to the second conduction-side pad electrode are drawn out from one side of the LED array to be bonded to the drive circuit.